Growth factors are substances, including peptides, which affect the growth and differentiation of defined cell populations in vivo or in vitro. Normal bone formation occurs during development, bone remodeling occurs in adult life, and bone repair occurs in order to preserve the integrity of the skeleton. Bone formation, remodeling and repair involve bone resorption by osteoclasts and bone formation by osteoblasts. Cell differentiation and the activity of osteoblasts and osteoclasts are regulated by growth factors. Thus, any interference between the balance in cell differentiation and resorption can affect bone homeostasis, bone formation and repair.
Osteoblasts are derived from a pool of marrow stromal cells (also known as mesenchymal stem cells). MSC are present in a variety of tissues and are prevalent in bone marrow stroma. MSC are pluripotent and can differentiate into chondrogenic or osteogenic cells including osteoblasts, chondrocytes, fibroblasts, myocytes, and adipocytes.
The induction of ectopic bone formation by demineralized bone matrix (DBM) has been described. (Urist, M. R.: Bone: Formation by autoinduction. Science 150:893-899, 1965; Urist, et al., Purification of bovine morphogenetic protein by hydroxyapatite chromatography, Proc. Natl. Acad. Sci. USA 81:371-375, 1984; Urist, M. R. Emerging concepts of bone morphogenetic protein. In Fundamentals of Bone Growth: Methodology and Applications, Boston C.R.C. Press, pp. 189-198, 1991.) Further, the properties of the partially purified protein fraction, bone morphogenic protein/non-collagenous protein (“BMP/NCP” or “BMP”s) have been described. (Urist, et al. Methods of Preparation and Bioassay of Bone Morphogenetic Protein and Polypeptide Fragments. In Methods in Enzymology. Vol. 146. New York, Academic Press, pp. 294-312, 1987; Urist, et al., Hydroxyapatite affinity, electrocution, and radioimmunoassay for identification of human and bovine bone morphogenetic proteins and polypeptides. In Development and Diseases of Cartilage and Bone Matrix. New York, Alan R, Liss, Inc., pp. 149-176, 1987.)
BMP/NCP was never purified to homogeneity, but other investigators have used similar starting materials to clone a number of recombinant “BMPs,” However several of these molecules have little or no osteogenic activity. “BMPs” and other osteogenic factors have been studied for use in clinical applications. However, the cost of using minimally effective dosages of BMP-7 (also known as OP-1), for example has been a limiting factor in clinical use. Therefore, effective and affordable compositions and methods are desired for clinical applications relating to bone.
Adjuvant therapeutics to enhance bone healing are important in many aspects of orthopedics, but are especially important for spinal fusion where prompt and thorough osteogenesis is critical. The most useful agents currently available include bone growth factors, such as bone morphogenetic proteins (BMPs). These proteins induce the recapitulation of endochondral bone formation among undifferentiated mesodermal cells. However, their usefulness is limited by expense and by local adverse effects such as unwanted ectopic bone formation and inflammatory responses associated with doses currently used tor spinal fusion procedures.
Furthermore, potential systemic adverse effects of high dose BMPs when to be used for longer fusions has not been fully determined. Therefore, a number of strategies are being developed to provide safer, less expensive and more efficacious adjuvant agents.
The use of BMP-2 as a bone generator for spinal fusion is gaining increased popularity. Recent studies have shown that it may be used effectively for both anterior and posterior fusion procedures through the whole spinal column. However, its high expense as well as the reported local side effects such as unwanted bone formation and dangerous swelling at the neck after the anterior cervical fusion procedure has prevented its extensive use for spinal procedures. Focus has now shifted to increase the effectiveness of BMP-2 while decreasing its dose and controlling the side effects. The main limitation is the need for better delivery systems that provide a sustained, biologically appropriate concentration of BMP-2 at the side of fusion bed. Delivery needs to be sustained, because BMPs have exceedingly short biological half-lives, usually the order of minutes or hours, rather than the days or weeks needed to stimulate a complete osteogenic response. For example, rBMP-2 has beers described as having a half-life of only a few hours.